Heisenberg exchange in magnetic monoxides

The superexchange intertacion in transition-metal oxides, proposed initially by Anderson in 1950, is treated using contemporary tight-binding theory and existing parameters. We find also a direct exchange for nearest-neighbor metal ions, larger by a factor of order five than the superexchange. This direct exchange arises from Vddm coupling, rather than overlap of atomic charge densities, a small overlap exchange contribution which we also estimate. For FeO and CoO there is also an important negative contribution, related to Stoner ferromagnetism, from the partially filled minority-spin band which broadens when ionic spins are aligned. The corresponding J1 and J2 parameters are calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and the Curie-Weiss temperatures, show appropriate trends, and give a reasonable account of their volume dependences. For MnO the predicted value for the magnetic susceptibility at the Neel temperature and the crystal distortion arising from the antiferromagnetic transition were reasonably well given. Application to CuO2 planes in the cuprates gives J=1220oK, compared to an experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental 230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory (1/28 revision

Gap opening in graphene by simple periodic inhomogeneous strain

Using ab-initio methods, we show that the uniform deformation either leaves graphene (semi)metallic or opens up a small gap yet only beyond the mechanical breaking point of the graphene, contrary to claims in the literature based on tight-binding (TB) calculations. It is possible, however, to open up a global gap by a sine-like one-dimensional inhomogeneous deformation applied along any direction but the armchair one, with the largest gap for the corrugation along the zigzag direction (~0.5 eV) without any electrostatic gating. The gap opening has a threshold character with very sharp rise when the ratio of the amplitude A and the period of the sine wave deformation lambda exceeds (A/lambda)_c ~0.1 and the inversion symmetry is preserved, while it is threshold-less when the symmetry is broken, in contrast with TB-derived pseudo-magnetic field models.Comment: 6 pages, 6 figures; (v2) added figures illustrating opening gap in Graphene mesh on BN, expanded analysis illustrating absence of pseudo-magnetic fields in deformed Graphen

Relative importance of crystal field versus bandwidth to the high pressure spin transition in transition metal monoxides

The crystal field splitting and d bandwidth of the 3d transition metal monoxides MnO, FeO, CoO and NiO are analyzed as a function of pressure within density functional theory. In all four cases the 3d bandwidth is significantly larger than the crystal field splitting over a wide range of compressions. The bandwidth actually increases more as pressure is increased than the crystal field splitting. Therefore the role of increasing bandwidth must be considered in any explanation of a possible spin collapse that these materials may exhibit under pressure.Comment: 7 pages, 4 figure

Dynamical Jahn-Teller Effect in Spin-Orbital Coupled System

Dynamical Jahn-Teller (DJT) effect in a spin-orbital coupled system on a honeycomb lattice is examined, motivated from recently observed spin-liquid behavior in Ba$_3$CuSb$_2$O$_9$. An effective vibronic Hamiltonian, where the superexchange interaction and the DJT effect are taken into account, is derived. We find that the DJT effect induces a spin-orbital resonant state where local spin-singlet states and parallel orbital configurations are entangled with each other. This spin-orbital resonant state is realized in between an orbital ordered state, where spin-singlet pairs are localized, and an antiferromagnetic ordered state. Based on the theoretical results, a possible scenario for Ba$_3$CuSb$_2$O$_9$ is proposed.Comment: 13 pages, 8 figures, to be published in PR

Origin of electron-hole asymmetry in the scanning tunneling spectrum of Bi2Sr2CaCu2O8+δBi_2Sr_2CaCu_2O_{8+\delta}

We have developed a material specific theoretical framework for modelling scanning tunneling spectroscopy (STS) of high temperature superconducting materials in the normal as well as the superconducting state. Results for $Bi_2Sr_2CaCu_2O_{8+\delta}$ (Bi2212) show clearly that the tunneling process strongly modifies the STS spectrum from the local density of states (LDOS) of the $d_{x^2-y^2}$ orbital of Cu. The dominant tunneling channel to the surface Bi involves the $d_{x^2-y^2}$ orbitals of the four neighbouring Cu atoms. In accord with experimental observations, the computed spectrum displays a remarkable asymmetry between the processes of electron injection and extraction, which arises from contributions of Cu $d_{z^2}$ and other orbitals to the tunneling current.Comment: 5 pages, 4 figures, published in PR

Modeling of gas adsorption on graphene nanoribbons

We present a theory to study gas molecules adsorption on armchair graphene nanoribbons (AGNRs) by applying the results of \emph{ab} \emph{initio} calculations to the single-band tight-binding approximation. In addition, the effect of edge states on the electronic properties of AGNR is included in the calculations. Under the assumption that the gas molecules adsorb on the ribbon sites with uniform probability distribution, the applicability of the method is examined for finite concentrations of adsorption of several simple gas molecules (CO, NO, CO$_2$, NH$_3$) on 10-AGNR. We show that the states contributed by the adsorbed CO and NO molecules are quite localized near the center of original band gap and suggest that the charge transport in such systems cannot be enhanced considerably, while CO$_2$ and NH$_3$ molecules adsorption acts as acceptor and donor, respectively. The results of this theory at low gas concentration are in good agreement with those obtained by density-functional theory calculations.Comment: 7 pages, 6 figure

Forming simulation of a thermoplastic commingled woven textile on a double dome

This paper presents thermoforming experiments and FE simulations of a commingled glass-PP woven composite on a double dome geometry, with the aim of assessing the correspondence of predicted and experimental shear angles. Large local deformations - especially in-plane shear, i.e. relative rotation between the two yarn families – occur when draping a textile on a three dimensional part and eventually unwanted phenomena like wrinkling or tearing may occur. The macroscopic drape behaviour of a weave is generally subdivided into: 1) The high tensile resistance along the yarn directions, expressed as non-linear stress-strain curves, and 2) The shear resistance, expressed as non-linear shear force versus shear angle curves. The constitutive model is constituted of a dedicated non-orthogonal hypo-elastic shear resistance model, previously described in [1, 2], combined with truss elements that represent the high tensile resistance along the yarn directions. This model is implemented in a user subroutine of the ABAQUS explicit FE solver. The material parameters have been identified via textile biaxial tensile tests at room temperature and bias extension tests at 200°. Thermoforming experiments are performed on a rectangular blank with the warp direction along the second symmetry plane of the tool, with a preheating temperature of 200°C, a constant mold temperature of about 70°C, and a blankholder ring. It was concluded that the shear angles were fairly well predicted for this particular case study, which could be expected in view of the fact that no wrinkles had formed during the thermoforming experiment

Damping of antiferromagnetic spin waves by valence fluctuations in the double layer perovskite YBaFe2O5

Inelastic neutron scattering experiments show that spin dynamics in the charge ordered insulating ground state of the double-layer perovskite YBaFe2O5 is well described in terms of eg superexchange interactions. Above the Verwey transition at TV = 308 K, t2g double exchange-type conduction within antiferromagnetic FeO2--BaO--FeO2 double layers proceeds by an electron hopping process that requires a spin flip of the five-fold coordinated Fe ions, costing an energy 5S^2 of approximately 0.1 eV. The hopping process disrupts near-neighbor spin correlations, leading to massive damping of zone-boundary spin waves.Comment: RevTeX, 4 pages, 4 figures, submitted to Phys. Rev. Let

Energy dependence on fractional charge for strongly interacting subsystems

The energies of a pair of strongly-interacting subsystems with arbitrary noninteger charges are examined from closed and open system perspectives. An ensemble representation of the charge dependence is derived, valid at all interaction strengths. Transforming from resonance-state ionicity to ensemble charge dependence imposes physical constraints on the occupation numbers in the strong-interaction limit. For open systems, the chemical potential is evaluated using microscopic and thermodynamic models, leading to a novel correlation between ground-state charge and an electronic temperature.Comment: 4 pages, 3 figs.; as accepted (Phys. Rev. Lett.

Ferromagnetism in the Mott insulator Ba2NaOsO6

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d^1 ferromagnetic Mott insulator with an ordered moment of ~0.2 Bohr magnetons per formula unit and TC = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.Comment: 5 pages, 5 figures, added reference